The aim of this study is to gather quantitative and qualitative data on the acute effects of ACTH, dibutyryl cyclic AMP, Ca2, and Mg2 ions on the ultrastructure and corticosterone production of dissociated adrenocortical cells of the rat. The effect of four agents will be tested on three cell systems. The agents will be ACTH, dibutyryl cyclic AMP, Ca2, and Mg2 ions. The systems will be trypsin-dissociated cells from untreated rats, from hypophysectomized rats, and from ACTH treated rats. After incubation of each cell system with either agent, the following parameters will be evaluated: corticosterone production, quantitative changes in the smooth endoplasmic reticulum (SER), mitochondria and lipid droplets, and qualitative changes in mitochondrial ultrastructure. It is hoped that the information obtained may help toward an understanding of the sequence of ultrastructural and biochemical events and their possible causal relationships in steroidogenesis.